In the lower end ethernet switch market, fixed configuration switches are widely used. Customers want the level of upgradability with these fixed configuration switches that has been previously achievable in chassis based configuration.
Seamless integration and upgradeability are achieved in Fixed Configuration switches with a Stack Port coupled to a Stack Bus, which is typically implemented as a ring for redundancy. In a chassis configuration, an arbiter can be centralized making it easy to enforce fair access for the line cards connecting to the switch fabric.
However, in a stackable system all the stack members on the stack may not have the same number of ports or bandwidth requirement. The number of ports or bandwidth requirement of a particular stack member in a stack is the load offered to the stacking bus.
It is highly desirable to have the stack bandwidth allocated in proportion to the offered load. As an example, a 48 port 100 Mbs stack member should get twice the share of stack bandwidth compared to 24 port 100 Mbs stack member. Stack members need a distributed system for insuring fair access to the stacking bus based on the offered load.
Shared ethernet allocates stack bandwidth using CSMA/CD, but the backoff time reduces the media utilization with a large number of nodes contending for the access.
FDDI allocates stack bandwidth by giving equal access to all the switches sharing the medium. This is a significant improvement over ethernet but this does not allocate the access opportunity in the ratio of the offered load.